Condensed Consolidated Statements of Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 322,725	$ 323,105
Costs and expenses:
Cost of sales — gas and purchased power (excluding depreciation shown below)	159,519	151,774
Operating and administrative expenses	58,940	51,409
Operating and administrative expenses — related parties	3,512	2,689
Depreciation	22,474	20,354
Other operating expense, net	1,206	9
Total costs and expenses	245,651	226,235
Operating income	77,074	96,870
Pension and other postretirement plans non-service income	412	(575)
Interest expense	(11,738)	(10,939)
Income before income taxes	65,748	85,356
Income tax expense	(15,860)	(17,053)
Net income	$ 49,888	$ 68,303